Hi Jiri,
On Tue, Mar 18, 2025 at 9:52 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Mon, Mar 17, 2025 at 10:24:24PM +0000, Sami Tolvanen wrote:
> > With CONFIG_GENDWARFKSYMS, __gendwarfksyms_ptr_<symbol>
> > variables are added to the kernel in EXPORT_SYMBOL() to ensure
> > DWARF type information is available for exported symbols in the
> > TUs where they're actually exported. These symbols are dropped
> > when linking vmlinux, but dangling references to them remain
> > in DWARF, which results in thousands of 0 address variables
> > that pahole needs to validate (since commit 9810758003ce
> > ("btf_encoder: Verify 0 address DWARF variables are in ELF
> > section")).
> >
> > Filter out symbols with the __gendwarfksyms_ptr_ name prefix in
> > filter_variable_name() instead of calling variable_in_sec()
> > for all of them. This reduces the time it takes to process
> > .tmp_vmlinux1 by ~77% on my test system:
> >
> > Before: 35.775 +- 0.121 seconds time elapsed ( +- 0.34% )
> > After: 8.3516 +- 0.0407 seconds time elapsed ( +- 0.49% )
>
> makes sense to me, I just can't reproduce the speedup
> could you please share your .config?
Sure, here's the config I used to repro this:
https://gist.github.com/samitolvanen/dca66a1a779861be27579f88c9b6ba5d
This is essentially x86_64 defconfig with GENDWARFKSYMS and
DEBUG_INFO_BTF both enabled. When building this config with gcc, we
end up with 0 address __gendwarfksyms_ptr variables in DWARF:
...
0x0001b5c6: DW_TAG_variable
DW_AT_name ("__gendwarfksyms_ptr_system_state")
DW_AT_decl_file ("../init/main.c")
DW_AT_decl_line (129)
DW_AT_decl_column (1)
DW_AT_type (0x0001b5dc "system_states *")
DW_AT_location (DW_OP_addr 0x0)
...
Note that this doesn't seem to happen when building with Clang.
Before commit 9810758003ce this resulted in pahole thinking all these
variables are in the .data..percpu section, which resulted in
btf_datasec_check_meta() failing with "Invalid offset" during boot.
pahole/next doesn't have this issue, but validating the 0 address
variables is unfortunately a bit slow when we have a lot of them.
Sami
